This invention relates to semiconductor devices. More particularly, it is concerned with junction field effect transistors.
The principal limitation of the maximum voltage capability in transistors is the mechanism of avalanche breakdown. Avalanche breakdown occurs when mobile electrons or holes in the semiconductor material are accelerated by an electric field which is sufficiently large that they gain a quantity of energy greater than the semiconductor bandgap before being scattered by other electrons or the lattice. In a scattering event with a valence band electron, the amount of energy is sufficient to raise the valence electron to the conduction band and thereby create an electron-hole pair. Each of these new charge carriers can themselves gain energy from the electric field and in turn create still more electron-hole pairs. In this way the current increases in an extremely short time to a point that is limited only by the series resistance of the circuit. In many instances the energy dissipated in the semiconductor by such large currents may be sufficient to cause thermal runaway and destruction of the device.
In semiconductor devices internal electric fields are found in the depletion regions of rectifying junctions, which may be either p-n, metal-semiconductor (Schottky), or metal-insulator-semiconductor. The electric field E.sub.max at which avalanche breakdown occurs is determined by ionization coefficients of electrons and holes which are specific to each semiconductor material. Given E.sub.max, the corresponding maximum voltage V.sub.max which can be supported without avalanche breakdown can be expressed for planar junctions as EQU V.sub.max =(E.sub.max.sup.2 e.sub.s /2qN.sub.d)-V.sub.b,
where e.sub.s is the semiconductor permittivity, q is the electronic charge, N.sub.d is the net dopant concentration, and V.sub.b is the built-in junction potential, which will normally be negligible compared to V.sub.max.
Because V.sub.max is inversely proportional to N.sub.d, it is clear that achieving a high breakdown voltage requires a low carrier concentration. However, a low carrier concentration means high resistivity, and so limits the current densities which can be transported without unacceptable heating. Therefore, conventional transistor technology permits either high carrier concentration, high current density devices operating at low voltage, or low carrier concentration, low current devices operating at high voltage, but not high current density, high voltage devices. Conventional high power transistors are made with low carrier concentration material but must use large junction areas in order to pass large currents. However, their substantial series resistance results in significant power dissipation, and together with the large capacitance due to the large junction area and minority carrier storage and transit time effects, limits their frequency response.